1. Introduction:
Since its discovery, buckminsterfullerene[1 ] has been considered as an important
precursor for several chemical compounds which have wide-scale
applications [2-7 ], ranging from material
science to biological sciences [8-10 ].
The functionalization of the fullerene cages including polymeric
derivatives has sorted out some of the problems associated with low
solubility and miscibility of corresponding C60 systems[11 ].In the past decades, a large number
of fullerene derivatives have been synthesized and characterized with
desirable applications [12-15 ], such as
photoelectric conversion [16 ], magnetic
resonance imaging [17 ], cancer therapy[18 ] and so on. Among the experimental
and theoretical methods implemented for fullerene modifications,
Diels-Alder cycloaddition reaction[19-23 ] on the fullerene surface and
encapsulation of atom/metal cluster/small molecules into the hollow cage
of fullerene [24-27 ] are drawing great
scientific attention. In this aspect, selective encapsulation of
fullerene cages is also observed to offer one of the key techniques in
their purification [28 ]. On the
Ih-symmetric C60, mainly two kinds of
C–C bond connectivity [29 ] have been
observed, namely [6, 6] and [6, 5]. The [6, 6] bond of
fullerene (C60) is found to exhibit much higher
reactivity towards Diels-Alder reactions with dienes[30, 31 ] than that of [6, 5] bond[32-35 ].
In recent decades, extensive studies on the chemical reactivity of
endohedral-metallofullerenes (EMFs) have got a new dimension. The EMFs
show enhanced reactivity not only towards Diels-Alder[36] but also to other essential
reactions necessary for varied utilization[37] . In this regard,
Li+ encapsulated fullerenes
(Li+@C60)[38,
39 ] are of great interest for both experimentalists as well as
theoreticians. According to the studies done by Ueno et
al. [40 ], a lesser HOMO-LUMO gap in
Li+@C60 compared to neutral
C60 is the principal reason for facilitating [4+2]
cycloaddition reaction, inducing significant changes in the frontier
orbitals. As a result, the Diels-Alder reaction of
Li+@C60 is 2400-fold faster than
neutral C60. However, the effect of counter anion in
Li+-encapsulated C60 during the
Diels-Alder reaction has also been explored[41 ].
Cui et al. [42 ] showed the
thermodynamic feasibility of cycloaddition reactions between CpH and
Ca2+@C60 as well as
M+@C60 (M = Li, Na, K, Rb, and Cs).
Their computational study inferred that encapsulated cations facilitate
DA reactions by altering distortion and interaction energies.
García-Rodeja and co-workers[43 ] also
explored the influence of varied ion-encapsulation
(Li+, Na+, K+,
Be2+, Mg2+, Al3+)
on the reactivity of the DA reaction between corresponding encapsulated
fullerene and 1,3-cyclohexadiene by employing Density Functional Theory
(DFT). Osuna et al. [44 ] reported
about the modulated reactivity and regio-selectivity in Diels-Alder
reaction of noble-gas-encapsulated fullerene systems. Recently, Wuet al. [45 ] observed that [6,5]
bond of calcium-encapsulated C60 molecule can be
successfully activated in the course of DA reaction based on charge
transfer from the metal atom to fullerene cage.
After reviewing these earlier reported works, it is quite evident that
the investigation of the reactivity of the EMFs in the context of
Diels-Alder (DA) reaction is a challenging field of exploration both in
terms of the experiment as well as theory. In our previous study[46 ], we have successfully explored the
energetics related to neutral C60 as well as mono- and
di-cation encapsulated EMFs towards DA reaction with 1,3-butadiene,
resulting in mono-functionalized products. But to the best of our
knowledge, the reactivity of neutral C60 as well as
encapsulated C60 towards Multi-Diels-Alder (MDA)
reaction beyond bis-functionalization[45 ] is yet to be revealed
computationally. Under these circumstances, we are going to investigate
the sequential MDA reactions on C60 fullerene surface
with 1, 3-butadiene computationally by employing DFT. Moreover, in the
present venture, we have extended our exploration to the titled reaction
procedure for Li+@C60 also to check
the effect of metal ion encapsulation on the reactivity of each step of
the MDA reaction. In order to get a clear idea about the bond
selectivity, both [6, 6] and [6, 5] connectivities on neutral as
well as charged EMF surfaces (Figure 1 ) are considered
separately for MDA reactions. Overall, our objective is to provide a
fundamental understanding of the reactivity of neutral
C60 towards MDA reactions and to enquire about the
effect of Li+-encapsulation in the reactions, as
mentioned earlier.